1. Field of the Invention
The present invention relates a hold-down device and, more particularly, to a hold-down device that has a force limiting feature and a positive locking and unlocking feature for inserting and extracting an electrical chassis from a rack and for placing a predetermined force upon the chassis.
2. Description of the Prior Art
The use of clamping devices to secure chassis to a rack or other support against shock and vibration is well known. The chassis may mount various kinds of equipment, such as electronic equipment, and may be mounted in a vehicle, aircraft or other environment that is subject to prolonged vibration and severe shock. The equipment for securing a chassis within a rack has been standardized in many cases. The standard chassis mount utilizes a hook mounted upon the front face of the chassis which is engaged by a clamping nut that fits over the hook. The clamping nut is mounted upon a threaded spindle that is typically pivoted upon the rack so that the nut, once engaged by the hook, places a force upon the chassis that forces the chassis into the rack and down against the rack shelf. Such a clamping assembly is shown in U.S. Pat. No. 3,212,746 which issued Oct. 19, 1965. This clamping assembly locks the clamping nut in place by moving a slide mounted on the nut, with an elongated slot therein, across the axis of the spindle so that the flat sides of the slot engage flats on the spindle to prevent rotation.
The clamping nut with its locking slide bar was replaced by later devices such as a device shown in U.S. Pat. No. 3,866,878 which issued Feb. 18, 1975, and which is assigned to Tridair Industries, predecessor of the present assignee. This device shows a unitary lock nut assembly which is keyed to the threaded spindle. Rotation of the nut brings the clamping nut into engagement with the hook on the chassis. Thereafter, further rotation causes the nut to stop but permits the camming surfaces on a drive knob and the clamping nut to slide over one another to engage ratchet teeth on the clamping nut and a follower which is keyed to a groove located in the outer periphery of the spindle. Unlocking is accomplished by rotating the drive knob in the opposite direction whereby the knob backs away from the locked nut for separating the ratchet teeth to create a free-spinning lock nut.
Another improvement is shown in U.S. Pat. No. 4,534,234 which issued Aug. 13, 1985, and which is assigned to the same assignee as the present invention. This hold-down device is provided with a slip clutch which permits the hold-down to exert a pre-determined load upon the hook attached to the chassis. Further, the hold-down device is provided with a specially designed hook connector which is not symmetrical, as in the earlier devices described above. The end of the hook connector facing the hook has a frustro-cylindrical shape which provides a ramp with a low point and a high point. At the lowest point of the ramp is located a deep slot which permits the hook connector to fit easily over the chassis hook. By rotating the hold-down device 180.degree., a shorter slot is brought into engagement with the hook for quickly initiating engagement therewith.
The hold-down devices described above are useful for retaining a chassis within a rack and, when tightened, for urging the chassis into its rack where a positive connection can be made between an electrical plug located on the outer, back surface of the chassis and an electrical connector mounted on the inner surface of the rack. Some of the prior art devices are provided with overload protection that prevents the hold-down device from being overly tightened which damages the chassis; see, for example, the device shown in U.S. Pat. No. 4,534,234. Other prior art devices are provided with a positive locking arrangement which prevents shock and vibration from loosening the hold-down device, as shown in U.S. Pat. No. 3,866,878. However, many of these devices require a groove or flat on the spindle thus weakening the spindle.